1. Field of the Invention
The present invention relates to a photolithographic process used in semiconductor manufacturing. More particularly, the present invention relates to a phase grating pattern used to produce modified illumination in a photo mask system and to a method of manufacturing a photo mask system having the phase grating pattern.
2. Description of the Related Art
A photolithographic process is used extensively to transfer pattern images in the manufacturing of semiconductor devices. However, the resolution of the photolithographic process must be increased to meet the current demand for semiconductor devices having higher and higher integration densities, i.e., to meet the demand for manufacturing semiconductor devices produced under smaller and smaller design rules. The resolution of the photolithographic process may be limitation by several factors, such as the wavelength of an exposure light source, the numerical aperture of a projection lens, the depth of focus, the process parameters, and the like. The need to increase the resolution of the photolithographic process, however, surpasses the limitations of current exposure apparatuses.
Attempts to overcome the limitations imposed by exposure apparatus on the resolution that can be achieved in a photolithographic process have included modifying the illumination systems of the apparatus and the introduction of phase shift masks. With respect to the latter, a grating has been added to a primary mask to diffract light transmitted through the primary mask, thereby modifying the illumination otherwise provided for the exposure process. Such grating-diffracted light causes a shift in the exposure energy illuminating the primary mask to provide an off-axis illumination of the primary mask. The resolution that is otherwise achievable using only the primary mask can thus be improved.
Such gratings up until now have been limited to simple line-space or simple mosaic types of gratings. For example, U.S. Pat. No. 6,057,065, issued May 2, 2000 to J. Brett Rolson, and entitled “Lithographic System Having Diffraction Grating and Attenuated Phase Shifters”, discloses simple line-space grating patterns. Such grating patterns are basically composed of simple line-space or square mosaic forms of patterns and are thus limited to providing modified illumination corresponding to a simple dipole or quadrupole. Simple dipole or quadrupole modified illumination can only achieve a limited increase in the resolution of the photolithographic process.
Accordingly, there is a growing interest in finding a way to further increase the resolution and depth of focus of an exposure process through the use of modified illumination.